环境工程专业英语文献中英双语版.docx

1、环境工程专业英语文献中英双语版Treatment of geothermal waters for production ofindustrial, agricultural or drinking waterDarrell L. Gallup Chevron Corporation, Energy Technology Company, 3901 Briarpark Dr., Houston, Texas 77042, USAReceived 14 March 2007; accepted 16 July 2007Available online 12 September 2007Abstr

2、actA conceptual study has been carried out to convert geothermal water and condensate into a valuable industrial, agricultural or drinking water resource. Laboratory and field pilot test studies were used for the conceptual designs and preliminary cost estimates, referred to treatment facilities han

3、dling 750 kg/s of geothermal water and 350 kg/s of steam condensate. The experiments demonstrated that industrial, agricultural and drinking water standards could probably be met by adopting certain operating conditions. Six different treatments were examined. Unit processes for geothermal water/con

4、densate treatment include desilication of the waters to produce marketable minerals, removal of dissolved solids by reverse osmosis or evaporation, removal of arsenic by oxidation/precipitation, and removal of boron by various methods including ion exchange. The total project cost estimates, with an

5、 accuracy of approximately 25%, ranged from US$ 10 to 78 million in capital cost, with an operation and maintenance (or product) cost ranging from US$ 0.15 to 2.73m3 of treated water. 2007 CNR. Published by Elsevier Ltd. All rights reserved.Keywords: Geothermal water treatment; Water resources; Desi

6、lication; Arsenic; Boron1. IntroductionWith the world entering an age of water shortages and arid farming land, it is increasingly important that we find ways of recycling wastewater. The oil, gas and geothermal industries, for example, extract massive amounts of brine and water from the subsurface,

7、 most of which are injected back into underground formations. Holistic approaches to water management are being adopted ever more frequently, and produced water is now being considered as a potential resource. In the oil and gas arena, attempts have been made to convert produced water for drinking s

8、upply or other reuses (Doran et al., 1998). Turning oilfield-produced water into a valuable resource entails an understanding of the environmental and economic implications, and of the techniques required to remove dissolved organic and inorganic components from the waters. Treatments of geothermal

9、water and condensate for beneficial use, on the other hand, involve the removal of inorganic components only.We have explored the technical and economic feasibility of reusing waters and steam condensates from existing and future geothermal power plants. Produced geothermal fluids, especially in ari

10、d climates, should be viewed as valuable resources for industry and agriculture, as well as for drinking water supplies. This paper presents the results of laboratory and field pilot studies designed to convert geothermal-produced fluids into beneficially usable water. The preliminary economics of s

11、everal water treatment strategies are also provided.2. Design layoutThe layout for the treatment strategies (units of operation) have been designed specifically for a nominal 50Mwe geothermal power plant located in an arid climate of the western hemisphere, hereafter referred to as the test plant. T

12、he average concentration of constituents in the produced water is shown in Table 1. The amount of spent water from the test flash plant is 750 kg/s. The potential amount of steam condensate that could be produced at the plant is 350 kg/s. Table 1 includes the composition of the steam condensate deri

13、ved from well tests. The six treatment cases considered in the study are given in Table 2, together with product flows and unit operations of treatment. Fig. 1 provides simplified schematic layouts of the unit operations for each case.3. Evaluation of treatment optionsIn this section the various ope

14、rations considered for each case are described.3.1. Arsenic removalThe techniques considered viable for removing traces of arsenic (As) from condensate or from water are ozone oxidation followed by iron co-precipitation or catalyzed photo-oxidation processes (Khoe et al., 1997). Other processes for

15、extracting As from geothermal waters (e.g. Rothbaum and Anderton, 1975; Umeno and Iwanaga, 1998; Pascua et al., 2007) have not been considered in the present study. In the case of the test plant, ozone (O3) would be generated on-site using parasitic power, air and corona-discharge ultra-violet (UV)

16、lamps, and iron in the form of ferric sulfate Fe2(SO4)3 or ferric chloride (FeCl3) that would be delivered to the geothermal plant. The photo-oxidation processes consist of treating the condensate or water with Fe2+ in the form of ferrous sulfate (FeSO4) or ferrous chloride (FeCl2), or with SO2 phot

17、o absorbers. The latter is generated from the oxidation of H2S in turbine vent gas (Kitz and Gallup, 1997).The photo-oxidation process consists of sparging air through the photo- adsorber-treated fluid, and then irradiating it with UV lamps or exposing it to sunlight to oxidize As3+ to As5+. In the

18、Fe photo-oxidation mode, the Fe2+ is oxidized to Fe3+, which not only catalyzes the oxidation reaction, but also co-precipitates the As. In the SO2 photo-oxidation mode, after oxidizing the As, FeCl3 or Fe2(SO4)3 is added to the water to precipitate the As5+ as a scorodite-like mineralTable 1Approxi

19、mate geothermal water and steam condensate compositions assumed in the studya Total dissolved solids.Table 2Summary of the six cases of geothermal fluid treatment to produce marketable watera On treatment of water, clays are produced at a rate of 7.4 ton/h.(FeAsO42H2O). In the laboratory and field p

20、ilot tests, the photo-absorber and UV dosages were varied to decrease the As concentration in geothermal fluids to below the detection limit of 2 ppb (Simmons et al., 2002). Residual As in the precipitate may be slurry-injected into a water disposal well or fixed/stabilized for land disposal to meet

21、 United States Environmental Protection Agency (USEPA) Toxicity Characterization Leach Procedure (TCLP) limits using special cement formulations (Allen, 1996).3.2. Ion exchangeStrong-base anion exchange resins have been shown to remove traces of As in geothermal fluids provided that the amorphous si

22、lica is decreased below its saturation point or the water stabilized against silica scaling by acidification. The ion exchange alternative to As removal by oxidation/precipitation has proven successful in reducing the concentrations of this element to below the limits set for drinking water standard

23、s. As part of the present study, laboratory and field columnar tests were successfully conducted with geothermal hot spring water containing 30 ppm As. Pre-oxidation of As3+ is required to achieve acceptable As removal by ion exchange. In these columnar tests, NaOCl and H2O2 were used to pre-treat t

24、he hot spring water to oxidize As3+ to As5+. Chloride-rich water, which had been treated with lime (CaOH2) and filtered to reduce amorphous silica to well below its saturation point, successfully regenerated the resin. In the field, and for simplicity of operation, we concluded that ozone/Fe co-prec

25、ipitation or catalyzed photo-oxidation would be preferred for water treatment over ion exchange as this would eliminate the need to purchase and transport additional chemicals. On the other hand, ion exchange is an attractive option for extracting As from condensate.Special ion-exchange resins have

26、proven successful in removing boron (B) from geothermal fluids (Recepoglu and Beker, 1991; Gallup, 1995). Hot spring water from the geothermal field, containing 25 ppm B, had its B content decreased to 1 ppm in a laboratory columnar test. The resin was regenerated with sulfuric acid (H2SO4). No dete

27、rioration in resin performance was observed up to 10 loading and regenerationcycles. Fig. 1. Flow chart of the basic unit operations involved in treatment cases 16.3.3. pH adjustmentThe majority of the cases considered in this study require adjustment to pH. Adding soda ash (Na2CO3) can increase the

28、 buffering capacity of the water and condensate. Soda ash or lime treatment can also be used to enhance precipitation of certain species. Purchased H2SO4, on-site generated sulfurous acid (H2SO3) or on-site generated hydrochloric acid (HCl) can be used to acidify waters to meet reuse requirements or

29、 to inhibit silica scaling (Hirowatari, 1996; Kitz and Gallup, 1997; Gallup, 2002). A number of geothermal power plants around the world utilize water acidification to inhibit silica scaling. Unocal Corporation commenced this practice of pH adjustment of hot and cold geothermal fluids in commercial

30、operations in the early 1980s (Jost and Gallup, 1985; Gallup et al., 1993; Gallup, 1996). In water acidification the pH is reduced slightly so as to slow down the silica polymerization reaction kinetics without significantly increasing corrosion rates.3.4. Cooling pondsIn this water processing optio

31、n, the water is cooled in open, lined ponds prior to injection or treatment for beneficial use. The flashed water is allowed to flow into the pond where it “ages” for up to 3 days; this is a sufficient length of time to achieve amorphous silica saturation at ambient temperature, which is assumed to

32、be below 20 C most of the year. Adjustment of the water pH to 8.00.5 with soda ash or lime enhances water desilication, resulting in undersaturation with respect to amorphous silica (Gallup et al., 2003). At 15 C, the solubility of amorphous silica in the water in our test field is predicted to be a

33、bout 90 ppm (Fournier and Marshall, 1983). In a large bottle, field water was adjusted from pH 7.2 to 8.1 with soda ash and allowed to cool to 15 C over a period of 90 min. The resultant dissolved silica Si(OH)4 concentration in the supernatant fluid was 54 ppm (undersaturated by about 40%).3.5. Filtra